Metal oxide nanowires are being investigated to make nanodevices and nanosensors. For example, In2O3 and SnO2 nanowires have been tested to form field-effect transistor gas sensors in recent year (D. H. Zhang, Z. Q. Liu, C. Li, T. Tamg, X. L. Liu, S. Han, B. Lei and C. W. Zhou, Nano Letters, 4 (2004) 1910; A. Kolmakov, D. O. Klenov, Y. Lilach, S. Stemmer and M. Moskovits, Nano Letters, 5 (2005) 667). This kind of gas sensors can detected ppb levels gases, such as NO2, NH3 est. In these researches, oxide nanowires are sonicated in a solution and little solution is dropped on substrates to make devices. Therefore, nanowires on devices show a random direction.
One of most popular methods to form oxide nanowires is vapor phase evaporation, which includes: heating the oxide powder to an evaporation temperature of the oxide powder for about 1 hour to about 2 hours at about 200 toor to about 400 torr in an atmosphere comprising argon; evaporating the oxide powder; and forming the oxide nanowires on a substrate (Z. L. Wang, Z. W. Pan, and Z. R. Dai, US Patent 2002/0094450 A1; Z. W. Pan, Z. R. Dai and Z. L. Wang, Science, 291 (2001) 1947). In order to evaporate oxide powders, heating temperature is usually set up to close to melting point of the oxide, for example 1000° C. The growth of oxide nanowires via vapor evaporation often involves a catalyst. A catalyst nanoparticle may present at one end of nanowire, which may affect the properties of the nanowire. A vacuum system is also required to maintain the pressure in a furnace. As high temperatures and vacuum is required for this method, the operation cost is also relatively high.
Another method of synthesis of oxide nanowire is thermal oxidation of a metal. It was shown that CuO nanowires could be formed on Cu foil by thermal oxidation in air. The formation of nanowires is mainly based on samples with small sizes. Generally, furnace temperature in the method of thermal oxidation (about 400-600° C.) is lower than that in vapor phase evaporation (over 1000° C.). Gas pressure in a furnace can keep 1 atm or lower. However, the length of nanowires formed by thermal oxidation is usually not uniform with low densities of nanowires.
Moreover, oxide nanowires formed by vapor phase evaporation or thermal oxidation often show a bending shape. Nanowire has a single crystal structure with spatial orientation. The bending structure of nanowire implies defects in the structures in nanowires.